A systematic investigation of the influences of migration, diffusion and convection on the growth modes of three-dimensional Ag electrodeposits on a Pt substrate was made using conventional electrochemical techniques combined with optical and scanning electron microscopic observations. The dependence of the growth mode on the dominant mass transport mechanism was established. Diffusion and migration influence the microscopic structure of the deposit. Migration dominates at the dendrite tip growth. Convection alters mainly the macroscopic growth of the deposit. Several transitions in the growth mode of Ag electrodeposits could be distinguished. One of these transitions was related to the initiation of dendrite formation, which appeared when a critical amount of electrodeposited Ag had been exceeded. The initiation of dendrite precursors was observed at borders and angles of the hexagonal Ag crystals formed first. These precursors continued to grow either as dendrites or lobular branched structures. Another transition which corresponded to a macroscopic shape transition of the deposit was dominated by free convection, whereas forced convection led to the development of a lobular rather than a dendritic deposit.